The present invention relates to compressors for compressing and discharging gas, and more particularly, to a compressor that varies displacement in accordance with the difference between the pressure of a discharge chamber and the pressure of a crank chamber, a control valve for controlling the pressure difference, and a method for varying the displacement of the compressor.
FIG. 20 shows a prior art compressor 200. An inclinable swash plate 201 is accommodated in a crank chamber 203. The displacement of the compressor 200 varies in accordance with the inclination of the swash plate 201. A control valve 202 controls the pressure of the crank chamber 203 to alter the inclination of the swash plate 201. The inclination of the swash plate 201 changes the stroke of pistons 204, which are retained in the compressor 200. There are two types of control valves 202, a self-controlled type and an externally controlled type.
A self-controlled type control valve detects the suction pressure of the compressor 200. The control valve automatically controls its position in accordance with the difference between the detected suction pressure value and a threshold pressure value. The threshold value is determined by the characteristics of a pressure sensing member (bellows), which is retained in the control valve. Accordingly, in a self-controlled type control valve, the threshold value cannot be changed when the compressor is operating.
In a externally controlled control valve, the threshold value can be changed when the compressor is operating. Typically, the externally controlled valve has an electromagnetic actuator and a controller 207. The electromagnetic actuator includes a solenoid 206 and other relevant parts (e.g., steel core). In the control valve, the solenoid 206 is arranged coaxially with a pressure sensing member. The controller 207 controls the electromagnetic actuator in accordance with data sent from various types of sensors (e.g., ambient temperature). The electromagnetic actuator is actuated to change the threshold value. The threshold value is changed to vary and optimize the displacement of the compressor under different conditions.
Since the prior art self-controlled control valve cannot change the threshold value, the displacement of a compressor using such a valve cannot be flexibly varied. Although the externally controlled control valve can change the threshold value in accordance with the conditions surrounding the compressor, the electromagnetic actuator, which includes the solenoid and other relevant parts, increases the size of the compressor and complicates the structure of the compressor. This increases the product costs of the compressor. Furthermore, an amplifier having a large electric capacity must be used to actuate the electromagnetic actuator, which is controlled by the controller. However, the employment of a compressor using a high-capacity amplifier in an automotive air conditioning system significantly increases the load applied to the vehicle.